Such systems are particularly useful for digitizing a drawing produced by hand on the writing medium, using a tool, by a human being. In this case, the tool is chosen from the group composed of a pencil and an eraser.
In this case, pencil denotes any tool that is directly manipulable by the hand of a human being in order to draw on the physical writing medium. In some cases, the pencil leaves a mark on the writing medium. The mark is then generally directly visible to the naked eye by a human being. In order to leave a directly visible mark on the medium, the pencil deposits a coloured liquid or solid on the writing medium. The coloured liquid is typically an ink or paint. The coloured solid is graphite, for example. In other cases, the pencil does not deposit a coloured liquid or solid on the writing medium. In this case, the pencil is often called a “stylus”. The plotted mark is then generally displayed on a screen.
In this case, eraser denotes any tool that is directly manipulable by the hand of a human being in order to draw on a physical writing medium a mark that, when it encounters a mark left by the pencil, erases the mark left by said pencil. The erasure includes digital data erasure of a mark recorded using a pencil. Moreover, if appropriate, the erasure may also be physical erasure of the mark on the medium. By way of example, the physical erasure of the mark on the medium may be the result of a chemical reaction and/or of a mechanical effect such as abrasion. Unlike the pencil, the mark drawn by the eraser is generally not visible to the naked eye.
In this case, “drawing” includes the event of running a tip of the tool over the writing medium in order to write or erase.
The writing medium is a sheet of paper, a canvas, a picture, a tablet or any other medium on which it is possible to draw.
In order to correctly plot the mark drawn by said tip on a writing medium, it is important to take into account the pressure exerted on the tip. By way of example, the pressure exerted on the tip allows precise detection of the existence of a point of contact between the tip of the tool and the writing medium. Now, every time the tip is no longer in contact with the writing medium, the plotted mark needs to be interrupted. It is also often desirable, in order to obtain a more realistic plot, to vary the width of the plotted mark or the luminous intensity of the plotted mark on the basis of the pressure exerted on the tip.
In this context, known systems for plotting the mark drawn on a writing medium have:                a tool, chosen from the group composed of a pencil and an eraser, said tool comprising:                    a body forming a means for grasping the tool with a hand of a user,            a tip intended to draw the mark to be plotted, said tip being movable by the reactive force exerted by the writing medium when said tip comes to rest on said writing medium, between:                        a prominent position and        a retracted position in which the tip is pushed inside the body more than in the prominent position,                    return means continually driving the tip towards its prominent position,            a first and a second, separate, permanent magnet, the first permanent magnet being mechanically linked to the tip so as to be moved translationally or rotationally, in relation to the second permanent magnet, as the tip is moved between its prominent and retracted positions,                        a device that is capable of recording successive positions for a point of contact between the tip and the medium in order to form the plot of the drawn mark, and of using a value of a physical quantity that is representative of the pressure exerted on the tip of the tool in order to control or complete said recording.        
By way of example, such a system is disclosed in the patent application WO 02/043045 or the application US2003/095115A.
In this prior art patent application, the position of the tip of the pencil is determined by measuring, using trigonometry, the position of a permanent magnet housed inside said pencil. This involves the use of three triaxial magnetometers arranged on the writing medium. This system is particularly advantageous in that, in order to locate the pencil, the latter is solely equipped with permanent magnets and therefore does not need to be powered from a power source. Such a pencil that does not need to be powered is termed “passive”. However, in the application WO 02/043045, when the inclination of the pencil is modified, this brings about modification of the amplitude of the magnetic field measured by the triaxial magnetometers, which translates into modification of the plotted position of the tip of the pencil. Thus, modifying the inclination of the pencil in the system of WO 02/043045 can bring about the appearance of an unwanted space in the mark plotted by said system.
Moreover, in order to complete the plot of the mark on the basis of a physical quantity that is representative of the pressure exerted on the tip, and not simply to detect a point of contact, the application WO 02/043045 teaches, with reference to FIGS. 14 and 15, that it is necessary to incorporate into the pencil a pressure sensor and a transmitter for transmitting the measured pressure. Thus, as soon as there is an attempt to determine the pressure exerted on the tip of the pencil, the pencil can no longer be “passive”.
It will also be noted that, furthermore, the application WO 02/043045 claims that a passive pencil equipped with two permanent magnets can be used to detect the instant at which the tip of the pencil comes into contact with the writing medium while preserving a passive pencil. More precisely, in the embodiment in FIG. 6, the application WO 02/043045 states that the contact between the tip and the writing medium causes the first permanent magnet to be placed on the second permanent magnet so that the first and second permanent magnets now form only a single large permanent magnet that radiates a magnetic field of much greater intensity. In FIG. 6, the first and second permanent magnets bear the references 62 and 64, respectively. It is this variation in the intensity of the radiated magnetic field that is measured and allows a point of contact to be detected. However, this embodiment does not work. The reason is that the increase in the magnetic field is due to the fact that the magnet 64 is going closer to the plane of the sheet and therefore to the triaxial magnetometers. In point of fact, the same variation in the amplitude of the magnetic field can be obtained by modifying the inclination of the pencil in relation to the plane of the sheet. It is therefore not possible to discern a variation in the measured amplitude of the magnetic field that is caused by the coming-together of the magnets 62 and 64 from a variation in this measured amplitude that is caused by the inclination of the pencil. Thus, in practice, the embodiment described in FIG. 6 therefore cannot be used to detect a point of contact between the tip of the pencil and a sheet.
The prior art is likewise known from: US2013/009907A1.